1. Field of the Invention
This invention relates generally to devices for general circuit protection including electrical distribution and motor control applications, more particularly to simple, reusable, potentially low cost devices that can be tailored to a plurality of applications, and most particularly to current limiting devices for relatively high power applications where the system voltage is equal to or greater than 100V and the short-circuit current is equal to or greater than 100 A utilizing an electrically conductive composite material and an inhomogeneous distribution of resistance structure.
2. Description of the Related Art
There are numerous devices that are capable of limiting the current in a circuit when a short-circuit occurs. One current limiting device presently being used includes a filled polymer material which exhibits what is commonly referred to as a positive-temperature coefficient of resistance ("PTCR" or "PTC") effect. The unique attribute of the PTCR effect is that at a certain switch temperature the PTCR material undergoes a transformation from a more conducting material to a more resistive material. In some of these prior current limiting devices, the PTCR material, typically polyethylene loaded with carbon black, is placed between pressure contact electrodes.
In operation, these prior current limiting devices are placed in the circuit to be protected. Under normal circuit conditions, the current limiting device is in a highly conducting state. When a short-circuit occurs, the PTCR material heats up through resistive heating until the temperature is above the switch temperature. At this point, the PTCR material resistance changes to a high resistance state and the short-circuit current is limited. When the short-circuit is cleared, the current limiting device cools down to below the switch temperature and returns to the highly conducting state. In the highly conducting state, the current limiting device is again capable of switching to the high resistance state in response to future short-circuit events. Examples of patents which disclose PTCR materials include U.S. Pat. Nos. 5,382,938 and 5,313,184, and European Patent No. 0 640 995 A1.
Current limiting devices based on the PTCR effect are typically designed for low power circuit applications, e.g., for a maximum current density of less than 10 A/cm.sup.2. Other current limiting devices are known for higher power applications. For example, commonly-owned U.S. Pat. No. 5,614,881 to Duggal et al. discloses a current limiting device comprising an electrically conductive composite material which includes an electrically conductive filler, two electrodes positioned adjacent to the composite material, an inhomogeneous distribution of resistance structure, and means for exerting compressive pressure on the electrically conductive composite material. In this device, the composite material does not rely on the PTCR effect to limit the current during a switching event.
The current limiting device described in the Duggal U.S. Pat. No. 5,614,881 is believed to effectively limit the current during a switching event by resistive heating of a high resistance thin layer followed by rapid thermal expansion and gas evolution from the binding material, which leads to a partial or complete physical separation of the current limiting device. The separation produces a higher overall device resistance to electric current flow which limits the flow of current through the short-circuited current path.
Although the current limiting device described in the Duggal U.S. Pat. No. 5,614,881 operates effectively to limit current, a high power switching event typically produces large mechanical stresses on the current limiting material. Thus, an advantageous feature of such a current limiting device is the ability to repeatedly withstand the high stresses caused by multiple high power switching events.